1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device having a ferroelectric capacitor that includes a ferroelectric film as its dielectric. In particular, the invention relates to a method of fabricating a semiconductor device having a ferroelectric capacitor, for recovering a deterioration of MOS (Metal Oxide Semiconductor) transistor characteristics occurring in fabrication processes.
2. Description of the Related Art
Conventionally, among important factors for semiconductor devices is an improvement of the deterioration in transistor characteristics resulting from in-process defects.
Transistor-including memories, at their final processes, are typically subjected to heat treatment in a hydrogen atmosphere. This heat treatment aims to terminate defects in the interfaces between the gate oxide films of the transistors and the substrates with diffused hydrogen so that the gate oxide films are lowered in interface state density for stable transistor characteristics.
Accordingly, this hydrogen-based heat treatment is the last to be performed in the fabrication processes. Nevertheless, since the oxide electrodes and ferroelectrics that form commonly-used ferroelectric capacitors are oxides, there has been a problem that the hydrogen-based heat treatment causes reduction of the ferroelectrics with deterioration in the characteristics.
More specifically, due to the application of the hydrogen treatment, the ferroelectrics, or the oxides used in the ferroelectric capacitors, are reduced to substances showing no ferroelectric-specific properties. This makes the ferroelectric capacitors unavailable. However, particularly to a so-called logic mixed ferroelectric memory in which both memory units composed of ferroelectric capacitors and logic circuit units are formed on a single chip, the heat treatment in hydrogen is an indispensable process for the sake of maintaining the characteristics of the logic circuit units.
For example, Japanese Patent Laid-Open Publication No. Hei 11-111930 discloses a method of fabricating a semiconductor device for preventing a deterioration of ferroelectric properties and an increase in leak current resulting from a heat treatment in an atmosphere of hydrogen-containing inert gas after the formation of an upper electrode (conventional example 1).
The technology of the conventional example 1 provides a method of fabricating a semiconductor memory device comprising: a MOS transistor to be a switching transistor formed on a semiconductor substrate; and a capacitor having a ferroelectric film as its dielectric and being electrically connected to the MOS transistor via a contact hole formed in a first insulating film. The semiconductor memory device is connected to another semiconductor device through wiring. In the method, between the formation of the dielectric film on a lower electrode of the capacitor and the formation of an upper electrode, a first heat treatment is performed in a hydrogen atmosphere at a temperature of 300-450xc2x0 C. so as to terminate defects in the interface between a gate insulating film of the MOS transistor and the semiconductor substrate. Moreover, the upper electrode of the capacitor is formed on the ferroelectric film and an interlayer insulating film is formed thereon, followed by the formation of contact holes and the formation of wiring for establishing connection between the MOS transistor and the other semiconductor memory device. Then, a surface protective layer is formed, and a second heat treatment of 300-450xc2x0 C. in temperature is performed in an oxygen atmosphere or in an atmosphere of mixed gas of oxygen and inert gas. This recovers the plasma damage introduced in the steps of opening the contact holes, installing metal wiring, and forming the surface protective film.
Moreover, for example, Japanese Patent Laid-Open Publication No. Hei 11-317500 discloses the application of heat treatment in hydrogen between the formation of multilayer wiring and the formation of ferroelectric capacitors (conventional example 2). A semiconductor device according to the conventional example 2 includes a plurality of pieces of metal wiring, a ceramic thin-film capacitor consisting of a lower electrode, a ceramic thin film, and an upper electrode, and a selecting transistor. Here, the selecting transistor is formed on a silicon substrate, a contact connected to this selecting transistor is formed, and the metal wiring is installed. Then, the semiconductor device is subjected to a heat treatment in a hydrogen-containing atmosphere, followed by the formation of the ceramic thin-film capacitor. Accordingly, via hole having tungsten plug need not be formed after the formation of the ceramic thin-film capacitor. This can prevent the ceramic capacitor from deteriorating in the step of tungsten CVD. In addition, the hydrogen-based heat treatment can decrease deterioration of the transistor.
In the technology of the conventional example 1, the hydrogen treatment (first heat treatment) is performed between the formation of the ferroelectric film and the formation of the upper electrode for the sake of preventing the deterioration of the ferroelectric film. This means a problem, however, that the hydrogen treatment inevitably deteriorates the properties of the ferroelectric film as long as the hydrogen heat treatment is performed after the formation of the ferroelectric film.
Meanwhile, the technology of the conventional example 2 has had a problem that defects occur in the step of forming the ferroelectric capacitor, thereby producing variations in the transistor characteristics.
An object of the present invention is to provide a method of fabricating a semiconductor device having a ferroelectric capacitor, in which transistor characteristics equivalent to those when the hydrogen-based heat treatment is performed at the last step can be obtained without a deterioration in ferroelectric properties.
The method of fabricating a semiconductor device having a ferroelectric capacitor according to the present invention is for fabricating a semiconductor device that includes a ferroelectric film as its dielectric. The method includes the steps of performing-a first heat treatment in a hydrogen-containing atmosphere to recover a defect occurring in a film forming step, forming the ferroelectric capacitor, and performing a second heat treatment in a nitrogen atmosphere to recover a defect occurring in the step of forming the ferroelectric capacitor.
According to the present invention, in the fabrication processes for forming a ferroelectric capacitor, the first heat treatment to precede the formation of the ferroelectric capacitor is performed in a hydrogen atmosphere, so as to recover defects that occur at the fabrication processes before the first heat treatment, in the oxide films of the MOS transistor or the like, or in the interfaces of the electrodes, oxide films, or the like. Then, the second heat treatment in a nitrogen atmosphere recovers defects that occur in the step of forming the ferroelectric capacitor. This makes it possible to recover defects occurring in the fabrication processes without deteriorating the ferroelectric capacitor by the hydrogen treatment.
Moreover, in the present invention, the first heat treatment step may follow the step of forming a MOS transistor on the surface of a silicon substrate, and the step of forming a first insulating film on this MOS transistor and forming a contact connected to a diffusion layer in the MOS transistor. The step of forming a lower electrode on the first insulating film, a ferroelectric film thereon, and an upper electrode thereon to form a ferroelectric capacitor, the lower electrode being connected to the contact, and the step of forming a second insulating film over the entire surface and forming a wiring for establishing connection between the contact and the upper electrode may be interposed between the first heat treatment step and the second heat treatment step. The step of forming a plurality of wiring layers between the contact and the ferroelectric capacitor may also lead the first heat treatment step.
Furthermore, the first heat treatment step may follow the step of forming a MOS transistor on the surface of a silicon substrate, the step of forming a third insulating film on this MOS transistor, and the step of forming a contact connected to a diffusion layer in the MOS transistor. Between the first heat treatment step and the second heat treatment step may be interposed the step of forming a ferroelectric capacitor consisting of a lower electrode on the third insulating film, a ferroelectric film thereon, and an upper electrode thereon, and the step of forming a fourth insulating film and forming a wiring for establishing connection between the contact and the upper electrode. The lower electrode of the ferroelectric capacitor may be connected to the contact.
The first and second heat treatments in the present invention may be performed at a temperature of 300-500xc2x0 C.